1) Field of the Invention
The present invention relates to RFID couplers and, in particular, to spatially selective couplers capable of selectively communicating with a targeted transponder from among a group of multiple adjacent transponders.
2) Description of Related Art
Radio frequency identification technology (“RFID”) allows wireless communication between an RF transceiver-reader or other similar device and one or more active or passive transponders. Active transponders are powered by a battery or other self contained power unit that is provided within the transponder and used as a DC source for conversion to RF energy. Alternatively, passive transponders draw energy from an electromagnetic field provided by an antenna or coupler of the RF transceiver. Various commands, data, or other information may be relayed between the RF transceiver and the transponders using a predefined “air interface” RF signaling protocol.
When multiple passive transponders are within range of RF transceiver's antenna they will each be energized and may simultaneously attempt to communicate with the transceiver, thus, potentially causing errors in “reading” or “writing” to one or more of the transponders. This is especially problematic in applications such as RFID printer-encoders.
In general, RFID printer-encoders are systems capable of printing and encoding RFID-enabled labels, tickets, tags, cards or other media “on demand.” These printer-encoders have a transceiver for communicating with the transponders embedded within or attached to individual media units. In various applications, it is highly desirable to present the individual media units on rolls or other structures in which the transponders are closely spaced. However, as referenced above, spacing passive transponders in relatively close proximity makes it more difficult to serially communicate with each transponder without simultaneously activating and communicating with neighboring transponders on other parts of the media roll.
To alleviate this problem, some techniques have focused on dispersing passive transponders embedded within or attached to carrier substrates such as labels, tickets, tags or other media supplied in bulk rolls, Z-folded stacks or other formats. For example, many applications provide passive transponders dispersed along media units or labels having sufficient length (or width) such that only one passive transponder is disposed within a given electromagnetic field at any even given time. Predictably, however, such dispersed configurations result in excessive media or carrier material costs. Further, winding or folding such dispersed labels into Z-folded stacks requires an increased packaging volume within the printer-encoder or other system as compared to standard length media rolls or stacks. Finally, widely dispersing passive transponders by providing longer labels or increased spacing between transponders may also slow overall printer-encoder throughput.
Anti-collision management techniques exist that allow near simultaneous reading and writing to numerous closely grouped transponders in a common RF electromagnetic field. However, such anti-collision management techniques often increase RFID system complexity, cost, and delay response. Furthermore, anti-collision management techniques are generally “blind” in that they do not recognize where a targeted passive transponder is physically located in the RF electromagnetic field. Systems incorporating such techniques would be less than desirable for printer-encoder applications as they have difficulty determining, for example, which passive transponder is located proximate the printhead of a printer-encoder. Accordingly, common objectives such as correlating information that is printed and encoded to a given label would prove highly difficult under standard anti-collision management techniques.
One way to prevent passive transponder read/write errors without using anti-collision management techniques is to electromagnetically isolate a specific transponder of interest from nearby transponders. Previously, electromagnetic isolation of physically adjacent transponders required passing transponders individually through RF-shielded housings or anechoic chambers for targeted exposure to an interrogating RF electromagnetic field. Unfortunately, use of such housings or chambers adds additional cost to the RFID system, tasks the often limited space requirements within the RFID system, and generally requires significant physical separation of the passive transponders. Furthermore, such housings or chambers are typically configured for media units of a particular type or size and such housing or chambers are generally not replaceable. Thus, operators which to print and encode media units of different types or shapes are often forced different printers/encoders.
Another known technique for selectively communicating with a specific transponder is controlling the strength or range of the electromagnetic field emanating from the transceiver's antenna. Typically, a transceiver is connected to a coupler or antenna. The transceiver generates the signal and the coupler is configured to broadcast the signal as an electromagnetic field. According to this technique, the coupler is configured to emit the signal within a limited range and directed to a specific targeted area. In theory, limiting the signal reduces the chance that additional transponders outside the targeted area will be activated. An exemplary short-range coupler is a one-half wavelength microstrip coupler, as generally described in U.S. patent application Ser. No. 10/981,967, hereby incorporated herein by reference. In general, the coupler 10 is based on a band pass filter (BPF) using ½ microstrip transmission line terminated by a 50 ohm resistor. Typically, as shown in FIGS. 1a and 1b, such couplers 10 includes a rectangular conductive strip 12 disposed upon a printed circuit. One end of the conductive strip 12 is connected to the transceiver and the other end is connected through a terminal load 16 to the ground plane layer 14. The conductive strip 12 is generally of sufficient width to produce adequate electromagnetic field strength for coupling to a targeted passive transponder.
However, conventional microstrip couplers and other couplers configured for short range communication are not without problems. For example, a coupler similar to that illustrated in FIGS. 1a and 1b may require a relatively high level of power to over an inherent low power efficiency and to ensure reliable activation and communication between a coupler and a transponder separated by even relatively short distances. The low power efficiency typically is caused from an inherent mismatch between the transponder's antenna impedance and the coupler's wave impedance. Impedance mismatch often stems from differing design criteria for the transponder and the coupler. For example, in many applications the transponder's antenna impedance may be set for maximum performance in far field electromagnetic transmissions (e.g., approximately 377 ohms) of the type expected during retail or industrial end use of the transponder. Alternatively, the coupler's wave impedance may be dictated by spatial selectively requirements for near field communications and further its RF port should match the standard impedance of the transceiver's RF port which is typically 50 ohms.
In non-printer-encoder applications impedance matching for a coupler based on a microstrip line has been addressed through the use of a conventional quarter wavelength transmission line 18, as shown in FIGS. 2a and 2b. However, to be effective, such quarter wavelength transmission lines generally must be relatively long for UHF band and not desirable in space-restricted systems such as UHF RFID enabled printer-encoder systems.